The invention concerns a circuit arrangement for the operation of a discharge lamp.
In the operation and upon ignition of a discharge lamp, high voltages can occur in a defective lamp and towards the end of a lamp's service life, which can lead to destruction of the corresponding electronic ballast if no suitable countermeasures have been provided. Customarily, a cutoff circuit is used to monitor lamp voltage. As soon as lamp voltage has been higher than a particular value for longer than a predetermined time, the cutoff intervenes and the lamp generator, which is generally a half bridge generator, is stopped.
Electronic ballasts with a cutoff circuit from the OSRAM GmbH company (product designation QT EC) are already known. A circuit arrangement corresponding to this state of the art is shown in FIG. 1. In this arrangement, the cutoff device is formed from a bistable flip-flop circuit consisting of two transistors T1 and T2 (thyristor equivalent circuit). The lamp voltage U.sub.L is fed via a voltage divider R4, R3 and a time-lag device R4, Ct to the base of one of the transistors of the bistable flip-flop circuit, which forms the trigger input of this circuit. When the bistable flip-flop circuit trips to its conducting or operation state, a cutoff transistor T3 stops the lamp generator, which is a half bridge generator in the example described, by suppressing the base drive of the lower transistor Tu of the half bridge generator via a diode D1. A starting capacitor Cs, which is located parallel to the switching path of the lower transistor Tu of the half bridge generator via a diode D4, is short-circuited via a diode D2 so that renewed starting attempts by the inverse rectifier are prevented.
This known circuit has two disadvantages in particular. In order that the oscillation of the half bridge generator can be dependably interrupted, the base of transistor Tu of the half bridge circuit must be connected with low impedance to ground. However, since the base voltage of transistor Tu of the half bridge generator can also show negative values, the base of this transistor Tu of the half bridge generator is connected via a diode D1 to the collector of the cutoff transistor T3. In this way, however, dependable disconnection is made more difficult, since the base of the lower transistor Tu is not directly connected to ground upon disconnection, but only via the diode path from D1 via T3.
A further disadvantage of the known circuit arrangement is that an energy source U.sub.v must be made available to provide the base current for the cutoff transistor T3. In known circuit arrangements, an electrolytic capacitor is provided for this purpose, which is loaded either using the trigger signal or via a pump circuit.
Therefore, the purpose of the present invention is to make a circuit arrangement available for the operation of a discharge lamp, in which the aforementioned problems are avoided.